This invention relates to the treatment of drill cuttings contaminated with oil for environmentally acceptable disposal, and more particularly to sequential treatment of the drill cuttings with an optional organic demulsifier, an acidification agent and alkaline earth for the purpose of rapidly removing the oil from the drill cuttings to obtain treated drill cuttings essentially free of oil.
Oil-based drill cuttings are generally regarded as controlled or hazardous waste. As such, the drill cuttings can be disposed of in two different ways: (1) decontamination treatment; or (2) hazardous waste controlled landfill. Hazardous waste is considered a threat to the environment due to the risk of surface and subsurface water pollution, as well as air pollution, interrupting the equilibrium of the ecosystem. The disposal of hazardous waste in controlled landfills is usually the last environmental option, since the problem is only transferred from one place to another and the ultimate solution is merely postponed for a later date.
There are several technologies available to treat hazardous wastes by different means. All of them have advantages and limitations depending upon the contaminant type and concentration, the matrix in which the contaminant is dispersed, and finally the locations at which the cuttings are generated and are to be disposed of, which can be the same or different. The handling and treatment costs, process time, contaminant locations such as ecologically protected areas, nearby water bodies, human residences, desert, etc. and finally the total treatment time, are all factors in selecting the best available technology.
Oil and gas exploration depend on drilling wells at different depths with different diameters throughout different geological strata with multiple lithological manifestations such as clay, rock, sand, empty underground salt mines, brine and water tables. Drilling requires a drilling fluid, also known as drilling mud, with various physical functions such as: (1) Cooling and lubrication of the drill bit; (2) Formation of a filter cake for temporarily ‘casing’ the wellbore; (3) Carrying the drill cuttings from the bit to the surface; and (4) Preventing blowout of reservoir fluids. The solid pieces of material cut by the bit, as the drilling advances, are known as drill cuttings. The drilling mud is a fluid of physical-chemical compounds with specific rheological characteristics to cover all the needs of the well as the different geological layers, depths and extreme pressure of natural fluids are met. There are two principal types of mud: (1) Oil-based mud (also known as inverse emulsion mud); and (2) Water-based mud. Their formulations vary according to the technology of each supplier and the general characteristics of each well in each field. These formulations are generally expensive, which is the reason for recirculating them. Before recirculation, their formulation must be adjusted to replace compounds lost during the process. The composition of many drilling muds typically includes the following compounds: (1) Bentonite; (2) Barite; (3) Diesel or other oil; (4) Polymers; (5) Sodium and potassium chlorides; and (6) Water. Water-based mud does not use diesel or oil but does use the chlorides; the inverse emulsion uses more diesel than water. As used herein, the term “oil-based mud” also includes synthetic muds that are sometimes classified separately even though they contain appreciable amounts of hydrocarbons, usually refined hydrocarbons instead of diesel. Because they do not contain difficult-to-dispose-of oil, water-based mud is sometimes used instead of the oil-based drilling fluids, even though the oil-based muds can be cheaper to use and can have operating advantages.
It is important to remember that, in all cases, the mud is a stable physical emulsion, necessarily so in order to prevent separation of its components that have different densities and other physical-electrical characteristics. Mud can be sticky and elastic, like gum, without losing its fluid qualities. As the contaminated oil-based drill cuttings lose water, they become stickier.
The mud is injected through the center of the drill string to the bit and exits to the surface in the annulus between the drill string and the wellbore, fulfilling, in this manner, the cooling functions and lubrication of the bit, casing of the well and, finally, carrying the drill cuttings to the surface. At the surface, the mud is separated from the drill cuttings to be reused, and the drill cuttings are disposed of, usually in controlled landfills.
The separation of the mud and drill cuttings is not perfect since the cuttings retain part of the drilling mud in concentrations that vary between 25 and in excess of 50 weight percent. Thus, drill cuttings can be considered hazardous waste, depending on the residual components of the mud and their concentrations. Environmental concerns demand that the drill cuttings showing contaminant characteristics, because of hazardous compound concentrations such as diesel, chlorides, polymers, etc., be handled and processed carefully before disposal into the environment. The best known prior art technologies for the treatment of inverse emulsion contaminated drill cuttings are: (1) Incineration; (2) Stabilization and Encapsulation; (3) Thermal Desorption; (4) Chemical Oxidation; (5) Biochemical Degradation; and (6) Controlled Landfills. The criteria used most often for selecting the best technology are: (1) Environmental reliability (environmental risk); (2) Specific environmental requirements, by legislation as well as geographical location; (3) Limitations presented by each technology (reliability of the equipment and processes); (4) Costs; (5) Process speed vs. cuttings generation speed; (6) Available space for treatment; (7) Characteristics of the final disposal site; and (8) Logistics. Encapsulation is seldom used because of the high risks involved since there is no guarantee of 100% encapsulation nor is there a guarantee that encapsulation will last over a long period of time under any environment at the final disposal site. Examples of encapsulation are seen in U.S. Pat. No. 4,913,586 to Gabbita; and U.S. Pat. No. 5,630,785 to Pridemore et al.
Biochemical degradation, as in U.S. Pat. No. 5,039,415 to Smith, requires constant supervision and control during the entire process, and this option is very slow and might take several years for treatment in each case. Controlled landfill is less and less attractive since the problem is not solved, and only changes the place and time for ultimate resolution.
Examples of incineration processes include U.S. Pat. No. 1,444,794 to Kernan; and U.S. Pat. No. 4,606,283 to DesOrmeaux et al. The main limitation for incineration lies in its operational costs, and the process control is also difficult since the tight stoichiometric operating ranges are hard to maintain when, in practice, contaminant concentrations are often variable. Moreover, the incineration process is energy intense because the entire matrix has to be heated to combustion temperatures, many constituents of which have high thermal coefficients. Furthermore, flexibility to set up incineration equipment in the field is low and the logistical costs are high.
Thermal desorption, as in U.S. Pat. No. 5,228,804 to Balch, U.S. Pat. No. 5,272,833 to Prill et al. and/or U.S. Pat. No. 5,927,970 to Pate et al., presents several limitations, including low thermal efficiency, poor process control, low flexibility, high investment cost, high operating cost, and low feasibility for in situ projects. The thermal efficiency of thermal desorption is even lower than incineration since the heating of the entire matrix has to be indirect, creating additional investment, maintenance and operational costs, with poor process control. The viscoelastic characteristics of the drill cuttings make processing difficult because of the tendency for the drill cuttings to stick to walls and other equipment surfaces, thus reducing the thermal transmission by effectively decreasing the inner diameter of the drum with less productivity and/or quality. Furthermore, thermal desorption requires additional treatment for the recovered gases, by condensation or other means of treatment, further increasing its cost.
Chemical oxidation is disclosed in U.S. Pat. No. 5,414,207 to Ritter, for example. In this approach, lime preconditioned with a hydrophobizing agent is blended with wet soil in an inert atmosphere and introduced to a decomposition vessel. Air is then introduced to the mixture to effect oxidation and/or hydrolysis of the oil contaminants. The main focus of this approach is to delay hydrolysis of the lime until well after the mixture is blended to favor oxidation/hydrolysis of the organic contaminants, and as a consequence the process is relatively slow and not continuous.